i have never been to a photography class, and cant afford to have courses, so where can i learn the general rules of photography? people are always talking about rules of 3rds...

...no clue what that is! i feel stupid when somebody asks me why i cropped a photo like i did (like there was no rules followed) and all i can say is, "cuz i thought it would look good that way". is there a site or software (thats not too spendy) where i can learn the things you'd learn in a class?